(205b) Mixing Immiscible Elements to Create a Library of Single Phase Ceramic Nanoparticles By Continuous Flame Synthesis

Ceramic solid solution nanoparticles incorporating two or more cations in a single homogeneous phase can have tunable features (e.g., active sites, vacancies, electronic structure) that can yield better performance than unary oxides in many fields, including catalysis. However, most reported ceramic solid solution materials combine similar elements (i.e. those with similar crystal structure, atomic diameter, valence, and electronegativity), such as alkaline-earth elements that form thermodynamically stable phases as bulk materials. This limitation to miscible elements dramatically limits the compositional space for new materials discovery. For example, incorporation of nickel into oxide phases that are used as catalyst supports would be desirable, because nickel is an inexpensive metal with broad catalytic activity for commercially important reactions. However, Ni-containing ceramic solid solutions and related phase diagrams have rarely been reported, because the conventional wet-chemistry methods such as co-precipitation performed at low temperature over long reaction times lead to separation of immiscible phases. In recent years, some nonequilibrium synthesis methods employing high temperature and short reaction time have been developed for mixing immiscible elements into a single ceramic or alloy phase, such as carbo-thermal shock, laser ablation, and spark discharge, but these methods often face barriers of scalability due to complexity, low yield, and high cost.

Flame aerosol processing is the most common method for scalable production of low-cost nanomaterials. Here, we reported a continuous, low-cost, and scalable flame synthesis method to overcome immiscibility and create a library of binary ceramic solid solutions with applications in catalysis. As shown in the figure, an aqueous precursor containing two metal salts is delivered to the flame reactor and atomized into microdroplets within which evaporation and reaction drives formation of nanoparticles. The reactor resistance time (~0.05 s) is much shorter than the time required for phase separation by solid-state diffusion, so the initial state of uniform composition in the droplet precursor can be retained in the product. Afterward, rapid quenching with diluting nitrogen prevents phase separation, finally forming a single binary ceramic solid solution. Here, we demonstrated the generality of this approach by mixing various elements including transition metals, noble metals, alkaline-earth elements, and others.

As a prototypical application, an amorphous Ni-Al ceramic solid solution is used to create a catalyst for dry reforming of methane. In this case, the initial metastable single phase undergoes nickel exsolution in a reducing atmosphere to create a nickel-decorated alumina catalyst with excellent catalytic activity and stability. Moreover, the droplet-to-particle conversion process by which the catalyst is formed results in a hollow micro- or nano-sphere morphology. Addition of a surfactant to the precursor solution allows templating of pores in the hollow shell which increase the surface area and make the interior of the shell accessible. The final hollow, porous, nickel-decorated alumina catalyst maintains over 90% CO₂ and CH₄ conversion for hundreds of hours at 800 °C when used as a dry reforming catalyst.